Corrosion-measuring device

ABSTRACT

This invention relates to an improved corrosion-measuring device in which at least a portion of a temperature sensitive voltage output means is located proximate to a corrosion test specimen. The corrosion test specimen is located in a circuit having a stable and normally constant input voltage. The extent of corrosion is determined by the decrease in current resulting from the increase in resistance of the test specimen as it corrodes over a period of time. The temperature sensitive voltage output means is at the same temperature as the test specimen and adjusts the input voltage to negate the effects of varying temperature on the current flowing through the test specimen.

United States Patent 65 CR, 65 TC, 105, 71 C, 65, 71; 204/1 T, 195,

" the test specimen.

[56] References Cited UNITED STATES PATENTS 3,067,386 12/1962 Freedman324/71 3,102,979 9/1963 Schaschl. 324/71 Primary Examiner-Herman KarlSaalbach Assistant Examiner-Marvin Nussbaum Attorneys-James R. Hoatson,Jr. and Philip T. Liggett ABSTRACT: This invention relates to animproved corrosionmeasuring device in which at least a portion of atemperature sensitive voltage output means is located proximate to acorroskin test specimen. The corrosion test specimen is located in acircuit having a stable and normally constant input voltage. The extentof corrosion is determined by the decrease in current resulting from theincrease in resistance of the test specimen as it corrodes over a periodof time. The temperature sensitive voltage output means is at the sametemperature as the test specimen and adjusts the input voltage to negatethe effects of varying temperature on the current flowing throughPATENTEUsmersn 3609549 /A/ l/E/V mks.-

Rudolf H. Haas/er Robe/I W. Sampson CORROSION-MEASURING DEVICE Thisinvention relates to an improved corrosion-measuring device in which atleast a portion of a temperature sensitive voltage output means islocated proximate to a corrosion test specimen. The corrosion testspecimen is located in a circuit having a stable and normally constantinput voltage. The extent of corrosion is determined by the decrease incurrent resulting from the increase in resistance of the test specimenas it corrodes over a period of time. The temperature sensitive voltageoutput means is at the same temperature as the test specimen and adjuststhe input voltage to negate the effects of varying temperature on thecurrent flowing through the test specimen.

BACKGROUND OF THE INVENTION It is frequently desirable to determine thecorrosion resistance of various materials through laboratory or fieldtesting. Such testing is useful to indicate the environments in which aparticular metal or alloy may be used satisfactorily and to determinewhether a metal, an alloy, or a protective coating conforms to aspecification requiring a certain performance in a specified corrosiontest.

Corrosion rates may be determined in different ways. Continuouselectrical and electrochemical measurements are among the mostinformative measurements. The most frequently used methods areresistance probe measurements and linear polarization measurements. Inresistance probe measurements the cross section of a metal resistanceprobe or test specimen in a corrosive environment gradually decreaseswith the result that the resistance of the metal probe increases. Thechange of resistance is therefore a measure of the progress ofcorrosion. The actual measurements may be made by including the testspecimen as a leg in a Wheatstone Bridge. The resistance of the testspecimen is determined by continuously rebalancing the bridge.Alternatively, the resistance can be determined by measuring the currentwhich flows through a test specimen connected in series to a constantpotential power source. Both forms of the resistance measurements aretemperature sensitive, however. In resistance measurements using aWheatstone Bridge, one arm of the bridge is usually made of the samematerial as the test specimen and is kept at the same temperature butisolated from the corrosive medium. This arrangement, although effectivein temperature compensation, may have a slow response to temperaturefluctuation.

Furthermore, continuous recording of resistance measurements by means ofa Wheatstone Bridge is complex and instrumentation usually expensive. Onthe other hand, continuous recording of the resistance of resistanceprobes is often desired and practical. It can easily be done if theresistance is measured in terms of current passing at a constantpotential, provided practical means for temperature compensation areavailable. Heretofore, no practical device has yet been implemented forcompensating for temperature fluctuations of a test specimen whencorrosion is determined by the measurement of current through a seriesconnected test specimen. Resistivity of a test specimen normallyincreases when temperature increases and decreases when temperaturedecreases because the resistance changes due to temperature fluctuationare usually greater than resistance changes due to corrosion. For thisreason, operation at a constant temperature has been essential.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto provide a resistance-probe-measuring device which automaticallycompensates for temperature fluctuations in the test specimen so that nocalculations are necessary to obtain the correct measurement of currentpassing through the test specimen as influenced by corrosion.

A related object of this invention is to eliminate the need for rigidtemperature control of the test specimen during the corrosion testperiod.

Another object is to provide a corrosion-testing instrument which willperform alternative modes of corrosion testing. These modes of corrosiontesting include a linear polarization measuring system and atemperature-compensated resistance probe. Alternative operation iseffected through manual or programmed automatic switches connected toappropriate electrical leads as will be hereinafter described.

Still a further object is to provide a corrosion-testing instrument withsimple means of continuous recording of the corrosion related electricalsignal.

The objects and advantages of this invention may be more fullyunderstood by reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of a resistanceprobe corrosion-measuring device utilizing the present invention.

FIG. 2 is the combination of a modification of the resistance-probecorrosion-measuring device of FIG. 1 used in alternative operation witha linear polarization corrosion-measuring device.

DETAILED DESCRIPTION OF THE DRAWINGS The invention illustrated in FIG. 1is a resistance-probe corrosion-measuring device having a stable,adjustable current source I, an electrically conductive corrosion testspecimen 3, and a current sensing means 22, all connected in series. Theinvention is an improvement over conventional resistanceprobecorrosion-measuring devices in that it further comprises a temperaturesensitive voltage output means having a portion proximately located withrespect to test specimen 3 and connected to adjustable current source 1,whereby current source 1 is adjusted to compensate for temperaturechanges at test specimen 3. The temperature-compensatedcorrosion-measuring device of FIG. 1 utilizes an electric circuit havinga control terminal 2. Control tenninal 2 is of no operationalsignificance but is merely a convenient reference point in the circuit.The stable, adjustable current source 1 is connected in the electriccircuit having an output 31 connected to control terminal 2. Inaddition, the adjustable current source 1 has a return input terminal 5and an error voltage input terminal 6. The corrosion test specimen 3 hasa first end 23 and a second end 24, and is connected in the electriccircuit with the first end 23 connected to the control terminal 2. Aspreviously mentioned, a current-sensing means 22 is connected in theelectric circuit. Adjustable current source 1, control terminal 2, testspecimen 3, and current-sensing means 22 are all connected in series inthe electric circuit. The current-sensing means 22 may be an ammeter, acurrent-sensing recorder, or other current-sensing device, but has beendepicted as an ammeter in the drawings. A first differential inputoperational amplifier 8 has an output 7 connected to the error voltageinput terminal 6 of the adjustable current source 1. Operationalamplifier 8 has a first input terminal 9 connected to the first end 23of the test specimen 3. Operational amplifier 8 also has a second inputterminal 10. A second differential input operational amplifier 11 has anoutput 12 connected to the second input terminal 10 of the firstdifferential input operational amplifier 8. Operational amplifier 11 hasa first input terminal 14 and a second input tenninal 13.

A voltage output means has a temperature sensitive portion 18proximately located with respect to the test specimen 3. The voltageoutput means is comprised of a thermistor 18, which is the temperaturesensitive portion, the direct current voltage source 19 connected to acommon point at 20, and an output terminal 32. The thermistor 18 is theonly portion of Operational amplifier 15 also has an output 17 connectedto the first input terminal 14 of operational amplifier l l.

A variable reference voltage source 21 is connected to the second end 24of the test specimen 3 and input terminal 13 of operational amplifier11. The ammeter 22 is located between the adjustable current source 1and the connection of the voltage source 21 to the second end 24 of thetest specimen 3.

A portion of the circuitry of FIG. 1 is a simplified form of aconventional potentiostat. That is, the circuitry comprising theadjustable current source 1, operational amplifiers 8 and 11 and thereference voltage source 21 and the connections to first and second ends23 and 24 of the test specimen 3 function in thesame manner as a simple,conventional potentiostat in a conventional resistance-probecorrosion-measuring device. The novelty of this invention resides in theincorporation of a voltage output means having a temperature sensitiveportion into the circuitry of an otherwise conventional potentiostat.

In the operation of this invention, there is a voltage V appearingacross the test specimen 3 due to the current flowing from theadjustable current source 1 through the test specimen 3. For theinstrument depicted to successfully measure corrosion of the testspecimen 3, V, must be maintained at a constant value with the exceptionthat V, must be adjusted to negate any changes in current at ammeter 22due to temperature variations of the test specimen 3. Since theresistance R of the test specimen 3 will normally increase with anincrease in temperature or decrease with a decrease in temperature, thevoltage V, must be increased or decreased accordingly so that there isno current change at ammeter 22 due to temperature variations within thethermally insulated jacket 4. Because this instrument compensates forcurrent changes due to temperature variations, it is not vitallyimportant to maintain an exact fixed temperature within the jacket 4.This greatly facilitates the measuring procedure and shortens the timerequired to obtain meaningful current measurements.

Each of the differential input operational amplifiers in the systemmodifies the difierence in voltage applied to its inputs. That is, thevoltage on one of the input terminals may be considered to be subtractedfrom the voltage of the other input terminal, and the resultantdifferential voltage times a gain factor appears at the operationalamplifier output. The thermistor 18 is merely a resistor which varieswith temperature. The thermistor may be constructed so that theresistance value either increases or decreases with an increase intemperature in the thermally insulated jacket 4. For the illustratedembodiment of the invention, a thermistor that increases in resistancevalue when the temperature within the insulated jacket 4 decreases isused. Conversely, when the temperature within insulated jacket 4increases, the resistance of thermistor 18 decreases.

Considering the case where the temperature within thermally insulatedjacket 4 increases, it can be seen that the resistance value of thethermistor 18 will decrease. Since the voltage source 19 is constant,the voltage drop across thermistor 18 will decrease since this voltagedrop is a product of current times resistance. This will result in alarger voltage appearing at the output terminal 32 of the voltage outputmeans. This larger voltage appears at the input terminal 16 ofoperational amplifier l and is amplified to appear at output terminal17. The voltage V appearing at output terminal 17 is larger than thevoltage at output terminal 17 that existed prior to the increase intemperature within thermally insulated jacket 4. The increase in voltageat terminal 17 also increases the voltage at input terminal 14. Thevoltage Vn y'at output terminal 12 of operational amplifier 1 l is theresult of an amplification factor of gain A times the difference involtage between the voltage V, at input terminal 13 minus the voltage Vat input terminal 14. That is, V A (V,V It can be seen that V decreasesdue to the increase in V Conversely, the voltage V, at output 7increases since the output voltage of operational amplifier 8 is theproduct of a constant gain A times the voltage V, at input terminal 9minus the voltage V at input terminal 10. That is V,=A,( V,,-V Since thevoltage V is decreased, the voltage differential is larger and thevoltage output V, at output 7 is larger. This larger voltage is appliedto the error voltage input terminal 6 of current source 1. The currentat output 31 of the variable current source 1 is the result of theproduct of a current gain A, times the voltage V, at the error voltageinput terminal 6. That is, I,=A V,. Since the voltage at input terminal6 is larger due to the temperature increase within jacket 4, the currentI, flowing through test specimen 3 increases; thus, the voltage Vappearing at control terminal 2 increases. The magnitude of the increasein voltage at the control terminal 2 is the voltage increment necessaryto increase the current through ammeter 22 by an amount equal to thedecrease in current at ammeter 22 resulting from the increase in theresistance value of the test specimen 3 due to the increase intemperature within the thermally insulated jacket 4.

It can be seen that the voltage at control terminal 2 will decrease byan amount calculated to offset an increase in current resulting from aresistance decrease when the temperature drops within thermallyinsulated jacket 4. As the temperature falls within thermally insulatedjacket 4, the resistance of test specimen 3 decreases. This tends toincrease the current flow through ammeter 22. However, since thetemperature sensitive voltage output means causes the voltage at controlterminal 2 to decrease by decreasing the current through the testspecimen 3, at static condition times current due to the resistancedecrease in test specimen 3 will be offset by an equivalent voltagedecrease at control terminal 2. More particularly, if temperature withinjacket 4 decreases, the voltage drop across thermistor 18 increases.This results in a decrease in the value of V Since V AA V,V V increases.Because V,.=A,( V,,V, V, decreases. As a result, 1 decreases sinceI,,=A,V,,.

The temperature compensation present in this invention will leave thechange in resistance in the test specimen 3 due to corrosion of the testspecimen 3 as the only uncompensated variable afiecting current. Therate of corrosion can be measured by the rate of decrease of current inammeter 22 over a period of time.

It is to be understood that each of the operational amplifiers depictedhas its own power source and may be of any conventional design used toachieve the specified functions as previously explained. Also, it shouldbe noted that there would be a slight change in circuitry if thermistor18 were replaced with a thermistor having a resistance value thatincreased with increasing temperature and decreased with a decreasingtemperature within thermally insulated jacket 4. With comparablemodifications, other temperature-sensing devices could also be used. Thetemperature-sensing devices illustrated are only examples of obtainingthe desired voltage signal. The use of other conventional means foraccomplishing the same result is contemplated within the scope of thisinvention. While the resistance values of thermistor l8 and testspecimen 3 have been described as varying in direct proportion totemperature changes, such a circuit design arises out of conveniencerather than necessity. Whatever the relationship of resistance values ofthermistor l8 and test specimen 3 to temperature, the invention will beoperable as long as the operational amplifiers are chosen so as toconvert the voltage change from the temperature sensitive voltage outputmeans into a voltage change at the control terminal 2 which will exactlycompensate for the resistance change of the test specimen 3 due totemperature.

The reference voltage source may be a battery or any other type ofdirect current source. For a given test specimen and for knownoperational amplifiers, the reference voltage source may be one having afixed potential, though as a practical matter it is depicted as avariable voltage source as in a conventional potentiostat so as toaccommodate different test specimens, operational amplifiers, andvoltage output means.

Because of the desirability of using either linear polarizationmeasurements or resistance probe measurements in determining corrosionrates, a combination of the resistance-probe corrosion-measuring deviceof this invention and a linear polarization instrument is illustrated inFIG. 2. A modified form of the corrosion measuring device of FIG. 1 isincluded in FIG. 2. Switches 25 and 27 connect adjustable current source1 and ammeter 22 in series alternatively with the corrosion testspecimen 3 and with electrodes 28, 29, and 30 of a conventional linearpolarization probe assembly 26. Linear polarization cell or probeassembly 26 has a control or working electrode 29, a reference electrode28, and an auxiliary electrode 30. The resistance-probecorrosion-measuring device of FIG. 2 has two modifications over that ofFIG. 1. A resistor 33 is included in the electrical circuit of FIG. 2.The purpose of this resistor 33 is to keep the current measurements atammeter 22 in the same order of magnitude regardless of whetheradjustable current source 1 and ammeter 22 are connected to the linearpolarization instrument 26 or to the corrosion test specimen 3. Resistor33 is necessary because a linear polarization cell has a much higherresistance than a resistance probe measuring device.

Another deviation from the circuitry of FIG. 1 is that the voltageoutput means of FIG. 1 is replaced by a thermocouple l8. Thermocouple 18is comprised of two wires of different metals joined in a junction attheir ends and producing a thermoelectric potential difference whenthere is a difference in temperature between the junction and theopposite ends of the dissimilar metal wires. The magnitude of the e.m.f.potential differential varies with the temperature difference betweenthe ends. Thermocouple 18 is constructed so that the potentialdifferential produced therein increases with an increase in temperaturewithin the thermally insulated jacket 4' and decreases with a reductionof temperature within the jacket 4 if the ambient temperaturesurrounding jacket 4 remains constant. In the modification of FIG. 2 itis necessary that the temperature surrounding the thermally insulatedjacket 4' be kept constant so that changes in ambient temperature do notaffect the output current at control terminal 2. The voltage at inputterminal 16 of operational amplifier l5 varies directly with the changein potential differential at thermocouple 18'.

The resistance-probe corrosion-measuring device of FIG. 2-

perfonns in the same manner as than of FIG. 1. In addition, the electriccircuit containing the adjustable current source 1, differential inputoperational amplifiers 8, 11, and and ammeter 22 may be connected to thelinear polarization instrument 26 for an alternative form of corrosionmeasurement. When he switches 25 and 27 are connected to the linearpolarization cell 26, the switch 34 is opened so that no unwantedinfluence from the voltage output means of the resistance-probecorrosion-measuring device is introduced into the system. When theswitches 25 and 27 contact the ends of the test specimen 3, the switch34 is closed so as to make operable the apparatus of this inventionproviding for temperature compensation.

The foregoing description and illustration of the embodiments of thisinvention are for purposes of illustration only, and no unnecessarylimitations should be construed therefrom as other modifications will beobvious to those skilled in the art of corrosion measurement.

We claim as our invention:

1. In a resistance-probe corrosion-measuring device having a stable,adjustable current source, an electrically conductive corrosion testspecimen, and a current-sensing means, connected in series, theimprovement comprising a temperature sensitive voltage output meanshaving a portion proximately located with respect to said test specimenand connected to said adjustable current source, whereby said currentsource is adjusted to compensate for temperature changes at said testspecimen.

2. The resistance-probe corrosion-measuring device of claim 1 furthercharacterized in that said current source is a potentiostat.

3. A temperature compensated corrosion-measuring device comprising:

a. an electric circuit having a control terminal;

b. an adjustable current source connected in said circuit and having anoutput connected to said control terminal and having a return inputterminal connected in said circuit and having an error voltage inputterminal;

c. an electrically conductive corrosion test specimen having first andsecond ends and connected in said electric circuit with said first endconnected to said control terminal;

d. A current-sensing means connected in said electric circuit in serieswith said test specimen;

e. A first differential input operational amplifier having an outputconnected to said error voltage input terminal of said adjustablecurrent source having a first input terminal connected to said first endof said test specimen and having a second input tenninal;

f. a second difierential input operational amplifier having an outputconnected to said second input terminal of said first differential inputoperational amplifier and having first and second input terminals;

g. a voltage output means having a temperature sensitive portionproximately located with respect to said test specimen and having anoutput terminal;

h. a single input operational amplifier having an input terminalconnected to said output terminal of said voltage output means andhaving an output connected to said first input terminal of said seconddifferential input operational amplifier; and,

i. a reference voltage source connected to said second end of said testspecimen and connected to said second input terminal of said seconddifferential input operational amplifier.

4. The combination comprising a resistance-probe corrosion-measuringdevice having a stable, adjustable current source, an electricallyconductive corrosion test specimen, and a current-sensing meansconnected in series, and a temperature sensitive voltage output meansproximately located with respect to said test specimen and connected tosaid adjustable current source, whereby said current source is adjustedto compensate for temperature changes at said test specimen, and alinear polarization corrosion-measuring instrument having electrodes,and switches connecting said adjustable current source and saidcurrent-sensing means in series alternatively with said test specimenand with said electrodes of said linear polarization cell.

2. The resistance-probe corrosion-measuring device of claim 1 furthercharacterized in that said current source is a potentiostat.
 3. Atemperature compensated corrosion-measuring device comprising: a. anelectric circuit having a control terminal; b. an adjustable currentsource connected in said circuit and having an output connected to saidcontrol terminal and having a return input terminal connected in saidcircuit and having an error voltage input terminal; c. an electricallyconductive corrosion test specimen having first and second ends andconnected in said electric circuit with said first end connected to saidcontrol terminal; d. A current-sensing means connected in said electriccircuit in series with said test specimen; e. A first differential inputoperational amplifier having an output connected to said error voltageinput terminal of said adjustable current source having a first inputterminal connected to said first end of said test specimen and having asecond input terminal; f. a second differential input operationalamplifier having an output connected to said second input terminal ofsaid first differential input operational amplifier and having first andsecond input terminals; g. a voltage output means having a tEmperaturesensitive portion proximately located with respect to said test specimenand having an output terminal; h. a single input operational amplifierhaving an input terminal connected to said output terminal of saidvoltage output means and having an output connected to said first inputterminal of said second differential input operational amplifier; and,i. a reference voltage source connected to said second end of said testspecimen and connected to said second input terminal of said seconddifferential input operational amplifier.
 4. The combination comprisinga resistance-probe corrosion-measuring device having a stable,adjustable current source, an electrically conductive corrosion testspecimen, and a current-sensing means connected in series, and atemperature sensitive voltage output means proximately located withrespect to said test specimen and connected to said adjustable currentsource, whereby said current source is adjusted to compensate fortemperature changes at said test specimen, and a linear polarizationcorrosion-measuring instrument having electrodes, and switchesconnecting said adjustable current source and said current-sensing meansin series alternatively with said test specimen and with said electrodesof said linear polarization cell.